The present invention relates to an improvement in the performance of a gas generator containing a pyrotechnic mixture in the form of granules or tablets, wherein the pyrotechnic mixture contains a nitrogen-containing fuel and an inorganic oxidizer.
One disadvantage of pyrotechnic mixtures within airbags inflators or seatbelt pretensioners, for example, includes poor moisture inhibition and therefore inconsistent performance. Relatively poor ignitability, poor sustained combustion, and low burn rates potentially cause poor inflator and/or pretensioner performance.
Conventional gas generant compositions such as those described in U.S. Pat. Nos. 5,035,757 and 5,139,588 are useful in vehicle occupant protection systems as applied within airbag inflator gas generators and in seatbelt pretensioners. However, nonazide gas generant compositions as exemplified therein may absorb moisture over time thereby inhibiting combustion performance. Furthermore, these compositions contain metal-containing oxidizers and thus produce relatively less gas and more solids when compared to other state of the art xe2x80x9csmokelessxe2x80x9d gas generants.
xe2x80x9cSmokelessxe2x80x9d gas generant compositions, such as those described in U.S. Pat. Nos. 5,872,329, 5,501,823, 5,783,773, and 5,545,272 (herein incorporated by reference) may be generally defined as producing at least 90% by weight of gas and not more than 10% by weight of solids upon combustion of the gas generant composition. These compositions have little, if any, metal-containing gas generant constituents and are also useful in vehicle occupant protection systems. However, nonazide compositions as exemplified therein may absorb moisture over time thereby inhibiting combustion performance.
Furthermore, to be useful in actuating vehicle occupant restraint systems, the formulations must ignite readily. xe2x80x9cSmokelessxe2x80x9d gas generants are often difficult to ignite and this sometimes results in inconsistent performance of an airbag inflator, for example. Finally, certain xe2x80x9csmokelessxe2x80x9d gas generants (i.e. reduced solid combustion products) exhibit reduced combustion sustenance; it is believed that reducing the metal containing compounds (and thereby reducing the combustion solids) also inhibits the burn characteristics of the composition. As a result, the composition may not fully burn and therefore may not provide the required performance.
The above-referenced problems are solved by coating any given gas generant composition with silicone thereby resulting in a moisture barrier, improved burn characteristics, and/or relatively more gas upon combustion.
The gas generant compositions contain one or more fuels, at least one oxidizer, and if desired, other additives well known in the art. In general, compounds that function primarily as binders are not required or used in the gas generant compositions described herein. Therefore, elastomeric, rubber, or silicone binders are not combined or mixed into the gas generant composition. One of ordinary skill will appreciate, however, that the silicone coating functions not as a binder but as a moisture inhibitor, as an auxiliary fuel, and as an ignition and/or combustion aid.
Stated another way, the use of a silicone coating, polydimethylsiloxane (PDMS) for example, results in reduced moisture retention, a greater percentage of gas combustion products per gram of a given coated gas generant composition, and an improved sustained combustion as compared to exemplary uncoated xe2x80x9csmokelessxe2x80x9d gas generant compositions.